Toxic arsenic remaining in thousands of destroyed Japanese chemical weapons sits in storage at a Chinese armed forces compound in the city of Nanjing, located in eastern China, Kyodo News reported on Thursday.

The poisonous substance — now sealed in plastic packages vulnerable to local flooding — is the end-product of an effort by Japan to eliminate chemical munitions that the nation had abandoned around Nanjing at the end of World War II, according to an insider involved in ties between Beijing and Tokyo. China has seen a number of demands for the material to go to the homeland of its former colonial occupier.